Add zsh completion for kubectx subcommand (-d) (#178)

- Add basic completion for subcommand -d.
  Note: Kubectx will suggest all available contexts.

- References:
  - http://zsh.sourceforge.net/Doc/Release/Completion-System.html#Completion-System
  - https://github.com/zsh-users/zsh-completions/blob/master/zsh-completions-howto.org

completion/kubectx.zsh

@@ -2,11 +2,17 @@
 
 local KUBECTX="${HOME}/.kube/kubectx"
 PREV=""
+
+local all_contexts="$(kubectl config get-contexts --output='name')"
 if [ -f "$KUBECTX" ]; then
     # show '-' only if there's a saved previous context
     local PREV=$(cat "${KUBECTX}")
-    _arguments "1: :(-
-        $(kubectl config get-contexts --output='name'))"
+
+    _arguments \
+      "-d:*: :(${all_contexts})" \
+      "(- *): :(- ${all_contexts})"
 else
-    _arguments "1: :($(kubectl config get-contexts --output='name'))"
+    _arguments \
+      "-d:*: :(${all_contexts})" \
+      "(- *): :(${all_contexts})"
 fi